1. Field of the Invention
The present invention relates in general to the field of microphones and tablet personal computers and, more particularly, to a plurality of microphones possibly arranged in the tablet (i.e., a microphone array) that has a directional polar response that continuously and automatically adapts to movement of an acoustic source.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
A microphone is generally recognized as an electro-acoustic transducer that converts an acoustic or sonic wave into an electrical signal. There are numerous types of transducers on the market, including piezoelectric transducers or transducers having a diaphragm that is pressure-sensitive.
Depending on the application, microphones can be chosen having different performance criteria. For example, a microphone can be embedded into a mouthpiece of a headset, with the mouthpiece less than a few inches from the speaker's mouth. In this example, the microphone is said to be placed in a “near field” setting. Alternatively, the microphone can be arranged at a pickup location on a table several feet away. In this example, the microphone is said to be placed in a “far field” setting. It is generally recognized that a near field setting is one in which the acoustic source is less than three inches from the microphone. See, Kozicki, “The Importance of the Microphone to the Pocket PC,” 2004, herein incorporated by reference.
The importance of knowing whether a microphone will be placed in a near field or far field setting is derived, in part, on whether the microphone will experience fairly significant noise. For example, a near field microphone placed within three inches of a speaker's mouth (i.e., the acoustic source) does not receive anywhere near the amount of background noise from the surrounding room compared to a far field microphone. For speech recognition systems, it is desirable that the signal-to-noise ratio (SNR) exceed 20 dB in order for speech to be recognized. If the microphone is placed in a near field setting, then the microphone can be formed as either a bidirectional, unidirectional, or omni-directional microphone. Conversely, if the microphone is placed in a far field setting, the microphone is generally a unidirectional microphone.
The primary difference between an omni-directional microphone and a unidirectional microphone is the directional polar response of those microphones. An omni-directional microphone can be a diaphragm-type microphone in contact with a sealed volume. Pressure upon that sealed volume from any direction around the diaphragm will cause the diaphragm to vibrate. Thus, the microphone picks up signals (and noise) anywhere around the microphone in a 360° directional polar response pattern. Conversely, a unidirectional microphone can be made in which the diaphragm or piezo-electric material is exposed on both sides to provide a pressure gradient difference between the front surface and the back surface. In addition, the acoustic wave from a forward direction will strike the back surface after it strikes the front surface, causing an internal acoustic time delay.
The time delay can be processed to bias the directional polar response of a unidirectional microphone toward a preferred direction. Moreover, there will be various null positions within the directional polar response near the side and back of the unidirectional microphone to effectively discard any background noise placed near the side and back of the microphone, and thus generally outside the forward-looking beam path angle. A unidirectional microphone thereby typically exhibits a “cardiod” directivity polar response pattern, whereby the SNR is greater within the forward beam path angle, and significantly lower outside that angle.
It is less cumbersome and thus more convenient for a user to interact with electronic systems that employ a microphone in a far field setting, with the microphone and acoustic source separated by more than three inches. Unfortunately, however, the acoustic source may periodically move while speaking into the microphone. For example, a user of a portable electronic system, such as a laptop computer, tablet personal computer (PC), or personal digital assistant (PDA), may move relative to a speaker mounted into the laptop, tablet PC, or PDA. If the acoustic source moves outside the desired beam path angle, then the SNR will be significantly jeopardized and, possibly, lower than the 20 dB needed to achieve adequate speech recognition. Thus, a system and method is needed to target and track an acoustic source relative to a microphone in a far field setting. Moreover, once the location of the source is identified, optimal SNR is achieved if the desired system and method can automatically and periodically update any movement of the source or changes in the source frequency, so that processing can occur to maintain the highest possible SNR no matter where the acoustic source is located, or the frequency of the acoustic signal.